1. Field
The disclosed concept relates generally to nuclear reactors and, more particularly, to core shrouds for nuclear reactors. The disclosed concept also relates to an associated method of assembling core shrouds.
2. Background Information
The primary side of nuclear reactor power generating systems which are cooled with water under pressure, comprises a closed circuit that is isolated from and in heat-exchange relationship with a secondary side for the production of useful energy.
FIG. 1 shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel 10 having a closure head 12 (also shown in FIG. 2) enclosing a nuclear core 14. A liquid reactor coolant, such as water, is pumped into the vessel 10 by pumps 16 through the core 14 where heat energy is absorbed and is discharged to a heat exchanger 18, typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam-driven turbine generator. The reactor coolant is then returned to the pump 16, completing the primary loop. Typically, a plurality of the above-described loops are connected to a single reactor vessel 10 by reactor coolant piping 20.
FIGS. 2 and 3 show simplified side elevation and top plan views, respectively, of the pressure vessel 10, and both show portions of the pressure vessel 10 in section view. The core 14 is comprised of a plurality of parallel, vertical co-extending fuel assemblies 22, only two of which are shown in FIG. 2 for ease of illustration. For purposes of this description, the other vessel internal structures can be divided into the lower internals 24 and the upper internals 26 (both shown in FIG. 2). In conventional designs, the lower internals 24 function to support and align the core and guide instrumentation, as well as direct flow within the vessel 10. The upper internals 26 restrain or provide a secondary restraint for the fuel assemblies 22, and support and guide instrumentation and core components, such as control rods 28. In operation, coolant enters the vessel 10 through one or more inlet nozzles 30, flows downward through an annulus between the vessel 10 and the core barrel 32, is turned about 180° in a lower plenum 34, passes upwardly through a lower core support plate 37 and a lower core plate 36 upon which the fuel assemblies 22 are seated, and through and about the fuel assemblies 22. In some designs the lower core support plate 37 and lower core plate 36 are replaced by a single structure. The coolant flow through the core and surrounding area 38 is typically large, on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tends to cause the fuel assemblies to rise, which movement is restrained by the upper internals 26, including a circular upper core plate 40. Coolant exiting the core 14 flows along the underside of the upper core plate 40 and upwardly through a plurality of perforations 42. The coolant then flows upwardly and radially to one or more outlet nozzles 44.
As shown in simplified form in FIG. 3, a core shroud 17 is positioned inside the circular core barrel 32, and includes a plurality of vertically extending plates 19 that convert the inner profile of the core barrel 32 to a stepped circumferential profile that generally matches the peripheral outline of the fuel assemblies 22 (shown in simplified form in FIG. 3) within the core 14. The simplified cross-section view of FIG. 3 also shows a thermal shield 15, which is interposed between the pressure vessel 10 and core barrel 32. Some plants have neutron pads in lieu of the thermal shield.
Typically, the plates 19 that form the stepped circumferential profile are substantially flat and abut at right angles at intersecting, corner, locations. As a result of machining and/or forming, some reactor vessel internals, however, include atypical corner joints. By way of example, these atypical corner joints can be characterized as being round for outside corner locations, being “key-like” (e.g., without limitation, having a groove) for interior locations and/or having relatively large pockets of open areas. Each atypical corner joint provides an area for flow to bypass the adjacent fuel assembly due to the low hydraulic resistance in these corners. In fact, flow calculations have shown a relatively high axial velocity in atypical core shroud corners. Among other disadvantages, this may result in unacceptable fuel rod vibration, which leads to fuel assembly grid-to-rod fretting, and may also cause elevated cross-flow velocities in this region.
There is, therefore, room for improvement in core shrouds and corner joints therefor.